This invention relates generally to integrated optical circuitry and, more particularly, to electrooptical modulators, which produce an optical output having an intensity proportional to an input electrical signal. In recent years, devices have been developed to make use of an electrooptical effect in which an electric field induces a change in the refractive index of an electrooptic crystal. An optical field propagating in a waveguide in the crystal is phase modulated by the induced change in refractive index. This effect can be employed in a variety of devices, such as modulators and switches.
There are two principal types of modulator/ switches in the prior art: the Mach-Zehnder interferometer and the directional coupler. In the Mach-Zehnder interferometer, an input waveguide is bifurcated into two parallel but relatively widely separated waveguide sections, which are then recombined into a single output waveguide. One of the parallel waveguide sections is influenced by an applied electric field, and the optical wave propagating in this section is phase modulated by the electrooptical effect. When the optical waves from the two parallel sections are recombined, they interfere and produce a resultant output wave whose intensity depends on the electric field applied to the device. For a zero-voltage input signal, the optical waves from the two sections combine constructively and produce a maximum intensity output. As the electrical input signal is increased in voltage, the electric field is increased and the interference of the two waves results in a smaller and smaller output intensity, until a zero output is reached.
Although the Mach-Zehnder interferometer is highly suited for use as a switch, in which the optical output may be switched to an off condition by the application of a suitable input signal, operation of the device as a modulator has at least two drawbacks. First, if the modulator is to have a linear characteristic, the device has to be biased with a direct-current (dc) bias voltage signal, to move its operating point to a linear portion of the device's output characteristic. The most desirable operating point for a modulator produces an output intensity of one-half the maximum output. Application of a dc bias signal necessitates additional components, and possibly a separate power supply. Even more important, the operating point is subject to "drift" over a period of time, due to optical damage caused by the photorefractive effect, and due to charge leakage between the electrodes of the device.
By way of background, in the photorefractive effect, an impurity in the waveguide, such as a ferrous ion (Fe.sup.2+), will absorb a photon of light and thereby generate free charge carriers in the form of an electron-hole pair. The charge carriers tend to accumulate on opposite sides of the waveguide, or on the top and bottom of the waveguide, depending on the orientation of the optical axis of the crystal. This charge accumulation produces an internal electric field that induces an unwanted change in the refractive index, through the electrooptical effect. As the optical power transmitted through the device is increased, the photorefractive effect increases, and imposes an upper power limit on the operation of the waveguide, beyond which the waveguide tends to operate less efficiently, and may suffer long-term optical damage. This damage is one factor that causes drift in the operating point of the device. Other factors, such as the wavelength of light, and the temperature, can also cause drift in the operating point, which in turn can degrade the device performance.
Another drawback of the Mach-Zehnder interferometer arises because the two parallel waveguide sections of the device must be widely spaced, to avoid any significant optical coupling between them. Because of this large waveguide separation, a dual electrode structure placed over the waveguides, and having an electrode spacing approximately equal to the waveguide spacing, is very inefficient. When a voltage is applied to the electrodes, the electrooptical effect influences the refractive indices of the waveguides, but inefficiency arises because of the smaller electric field, which results from the large electrode spacing. To increase the electric field, a smaller electrode spacing is required. However, since the electrode spacing is smaller than the waveguide spacing, only one waveguide can be subjected to the electric field. Thus the electrooptical effect influences the refractive index of only one of the waveguide sections. This is known as non-push-pull operation, in contrast to the push-pull characteristic of the directional coupler, to be discussed next, in which the electrooptical effect influences two waveguides in opposite directions, using an efficient dual electrode structure having a small electrode spacing that matches the waveguide spacing. In push-pull operation, therefore, the same voltage has practically twice the effect as in non-push-pull operation, since both waveguides are influenced by the same voltage equally, but in opposite senses.
The directional coupler includes two parallel waveguide sections that are spaced closely enough to be closely coupled, but are not otherwise connected together. An input optical signal is applied to one end of one of the waveguide sections and the output is derived from an opposite end of either one of the waveguide sections. The waveguide sections have associated electrodes for applying an electric field, which affects the sections in opposite senses. Unlike the Mach-Zehnder interferometer, therefore, the directional coupler operates in a push-pull manner. The waveguide section length is selected such that, when the electrical signal is zero, all of the optical input energy is coupled rom one waveguide section to the other. The output from one waveguide section is then zero, while the output from the other is at maximum intensity. As the electrical input signal is increased, the degree of coupling between the waveguide sections also changes. The output from one waveguide section increases from zero to a maximum, while the output from the other waveguide section decreases to zero. For operation as a modulator, the device must be dc biased to obtain an operating point on a linear portion of the operating characteristic, preferably at the half-maximum output point, sometimes known as the 3 dB operating point. Therefore, although the directional coupler has a desirable push-pull mode of operation, it still suffers from the principal disadvantage of the Mach-Zehnder interferometer, in that it requires a dc bias, which is subject to drift.
Accordingly, there is still a need for an electrooptical device operable as a switch or modulator, which avoids these disadvantages of the prior art. The present invention fulfill this need, as will become apparent from the following summary.